Recently, power generation machines such as construction working machines require performances of reduced fuel consumption, low environmental pollution, low noise, and the like, taking the global environment into consideration. In order to satisfy these requirements, a working machine such as a hydraulic shovel using a motor has appeared as a substitute for or a supplement for the hydraulic pump. In a working machine in which a motor is assembled, surplus kinetic energy that is generated from the motor is converted into electric energy, and the converted electric energy is accumulated in a capacitor or the like.
The capacitor is deteriorated due to long-term use, in which charging and discharging are repeated, or due to overcharging, excessive discharging, heat generation and the like. By measuring the internal resistance of the capacitor, its deterioration state can be determined (Japanese Unexamined Patent Publication No. 2007-155586).